Bleeding in the gastrointestinal tract is a major clinical problem. Bleeding may be acute and severe or it may be chronic and unnoticed (i.e. occult bleeding). The acute bleeding is a medical emergency that needs to be evaluated and treated immediately. Occult bleeding may be a sign of colon cancer and demands a careful evaluation to avoid missing a cancer in a curable stage. An initial and important determination in the medical management of either type of gastrointestinal bleeding is the localization of the bleeding site to either the upper or the lower gastrointestinal tract. During the Phase I portion of this project we developed a simple in vitro test for the analysis of blood in stool samples. This test was designed to determine, from a stool sample, whether bleeding was from an upper or lower gastrointestinal site. The objective of this Phase II project is to confirm the clinical accuracy and usefulness of this test with a large prospective group of patients with gastrointestinal bleeding in a clinical environment.